数组上的随机值

Question

I would like to generate a random value on an array. But each click, the array gain a new value, so I don't want to get a random value who have already a value on the array. In a nutshell, I need to random for the empty value of the array.

Here is a little part of the code. I have 9 values on my array, and at the beginning all of them are empty.

var gameCase = ['', '', '', '', '', '', '', '', ''];
var randomValue = gameCase[VALUE_EMPTY*][Math.round(Math.random()*gameCase.length)];

*Here is the part I don't really know how to do it.

Answer 1

There are few ways to approach this:

Method 1: Keep generating random index until it points to an empty value

var gameCase = ['', '', '', '', '', '', '', '', ''];
var randomIndex = Math.round(Math.random()*gameCase.length) % emptyCases.length;
var randomValue = gameCase[randomIndex];
// while we haven't found the empty value
while (randomValue !== '') {
    // keep on looking
    randomIndex = Math.round(Math.random()*gameCase.length % emptyCases.length);
    randomValue = gameCase[randomIndex];
}
// when we exit the while loop:
//     - randomValue would === ''
//     - randomIndex would point to its position in gameCase[]

Method 2: Have a 2nd array that keeps track of which indices of gameCase array have empty values

var gameCase = ['', '', '', '', '', '', '', '', ''];
var emptyCases = [0,1,2,3,4,5,6,7,8];

if (emptyCases.length > 0) {
    // generate random Index from emptyCases[]
    var randomIndex = emptyCase[Math.round(Math.random()*emptyCase.length) % emptyCases.length];
    // get the corresponding value
    var randomValue = gameCase[randomIndex];
    // remove index from emptyCases[]
    emptyCases.splice(randomIndex, 1);
}

Method #2 is more efficient in a sense because you don't have waste time to generate/guess a random index. For Method #1 you need a way to check if there are any empty values left in gameCase[] or else you might be generating/guessing forever in an infinite loop.

More: When you set values for gameCase[] you need to update emptyCases[] accordingly to accurately reflect the state of gameCase[] :

var gameCase = ['', '', '', '', '', '', '', '', ''];
var emptyCases = [0,1,2,3,4,5,6,7,8];

/* Update a value in gameCase[] at the specified index */
var setGameCaseValue = function(index, value) {
    // set value for gameCase[]
    gameCase[index] = value;

    if (value !== '') {    // we're setting a value
        // remove that index from emptyCases[]
        emptyCases.splice(emptyCases.indexOf(index), 1);   
    } else {    // we're setting it back to empty string
        // add that index into emptyCases[] that refers to the empty string in gameCase[]
        emptyCases.push(index);        
    }
};

setGameCaseValue(2, 'null');
// gameCase now has ['','','null','','','','','','']
// emptyCases now has [0,1,3,4,5,6,7,8]

setGameCaseValue(0, 'null');
// gameCase now has ['null','','null','','','','','','']
// emptyCases now has [1,3,4,5,6,7,8]

setGameCaseValue(5, 'null');
// gameCase now has ['null','','null','','','null','','','']
// emptyCases now has [1,3,4,6,7,8]

setGameCaseValue(7, 'null');
// gameCase now has ['null','','null','','','null','','null','']
// emptyCases now has [1,3,4,6,8]

See fiddle: http://jsfiddle.net/rWvnW/1/